Point-of-Entry ultraviolet water sterilization devices typically include a pressure vessel including a stainless steel cylinder enclosing a smaller concentric quartz light transmission tube. Within the quartz tube is a tubular discharge lamp emitting light with wavelengths typically centered around 254 nm, which is referred to as ultraviolet light or ultraviolet radiation. Ultraviolet (UV) light represents a section of the overall electromagnetic spectrum of light, extending from the blue end of the visible at about 400 nm to a region of about 100 nm.
Water to be exposed to the light from the lamp is passed between the interior surface of the steel cylinder and the exterior surface of the light transmission tube. One important limitation to this type of device is that the exterior surface of the quartz tube becomes fouled with inactivated biological contaminants in the water as well as minerals at least in part due to a photochemical reaction upon exposure to the light of the lamp. Due to the arrangement of the discharge lamp contained in a quartz sheath, the foulants collect on the exterior of the quartz tube. The foulants cause a reduced transmission of ultraviolet light through the light transmission tube, which results in a reduced efficacy of the device. This requires that the device be disassembled periodically for cleaning of the quartz tube to maintain its effectiveness.
To reduce maintenance and downtime, automatic wiping mechanisms have been tried on these systems with limited success. U.S. Pat. No. 5,266,280 discloses a system of radially mounted brushes that act to wipe the external surface of the quartz sheath. This costly mechanism is difficult to actuate and to provide adequate sealing for the power transmission shaft connected to the motor outside of the steel cylinder of the pressure vessel. Also described are a number of other similar external wipers, which have varying amounts of effectiveness. Cleaning the tube in this manner is cumbersome and compromises the irradiation dosage of the device because the gap between the outer wall of the quartz sheaf and the inner wall of the pressure vessel must be increased to accommodate the wiper resulting in a larger dosage gradient across the laminar fluid cross section.
Attempts have been made at providing means to conduct fluid through a quartz tube with UV emitting lamps arranged external to the quartz tube and with a wiper mechanism acting to clean the inside walls of the tube. These attempts have failed primarily due to material degradation. The ultraviolet light emitted by germicidal lamps causes degradation of some form in all polymer-based materials. Many components used in the construction of an internal automated wiper system must be polymer-based for friction reduction, manufacturability and sealing-performance reasons. U.S. Pat. No. 5,266,280 describes a number of UV resistant materials such as halogenated polyolefins, urethane, synthetic rubber and high-density polypropylene, all of which are susceptible to being degraded by UV light. Some polymeric materials resist UV degradation. However, when they are exposed to UV radiation centered around 254 nm, and are provided in contact with the fluid being sterilized, will leach small doses of volatile organic compounds. This defeats the purpose of purification. Materials certified by government organizations for use with potable water are tested without irradiation of UV light and many polymers in this category are found to be non-compliant after irradiation. For this reason it is unacceptable to simply claim compliance for potable water of materials used in irradiated locations of UV sterilization devices as is typically done.
U.S. Pat. No. 4,002,918 discloses a device of similar arrangement for the purpose of “the irradiation of fluids to initiate chemical reactions”, but describes wiper materials as being plastic, which are unacceptable for ultraviolet water sterilization applications. There is no mention of shielding materials from harmful ultraviolet light to prevent the leaching of volatile organic compounds, which has been observed to be a critical factor in the design of ultraviolet water sterilization devices.
Another area of concern is the tendency for heat to build up in the apparatus, which potentially leads to a reduction of reliability and efficacy of the unit.
There is a demand for a UV water purification device that combines superior performance and low maintenance costs. The present invention satisfies this demand.